This invention relates to reducing crosstalk between communications systems. The invention is particularly, but not exclusively, applicable to reducing NEXT (near end crosstalk) between twisted pairs of wires in telephone cables used historically for providing telephone service to subscribers and now being used increasingly to provide additional communications services, for example for data communications and computer network connections.
Reference is directed to U.S. patent application Ser. No. 08/640,705 filed May 1, 1996 in the names of J. B. Terry et al., entitled xe2x80x9cInformation Network Access Apparatus And Methods For Communicating Information Packets Via Telephone Linesxe2x80x9d, the entire disclosure of which is hereby incorporated herein by reference. This application, referred to below as the related application, describes and claims methods and apparatus which can be used in particular to facilitate remote access via conventional twisted pair telephone lines to computer networks such as the global computer information network which is generally known as the Internet and is referred to herein as the Network. The present invention is not limited in any way to the arrangements of this related application, but can be applied in a particularly convenient manner to such arrangements as is described later below.
Twisted pair public telephone lines are increasingly being used to carry relatively high-speed signals instead of, or in addition to, telephone signals. Examples of such signals are ADSL (asymmetric digital subscriber line), HDSL (High Density Subscriber Line, T1 (1.544 Mb/s), and ISDN signals. There is a growing demand for increasing use of telephone lines for high speed remote access to computer networks, and there have been various proposals to address this demand, including using DOV (data over voice) systems to communicate signals via telephone lines at frequencies above the voice-band.
The provision in the public telephone network of varied services using such diverse communications systems imposes a requirement that different and similar systems not interfere with one another. A predominant limiting effect in this respect is NEXT (near end crosstalk) between wire pairs within multiple-pair cable binder groups or between wire pairs within adjacent binder groups.
Allocations of wire pairs within telephone cables in accordance with service requests have typically resulted in a random distribution of pair utilization with few precise records of actual configurations. In addition, due to the nature of pair twisting in cables, and where cable branching and splicing occurs, a wire pair can be in close proximity to different other pairs over different parts of its length. At a telephone C.O. (central office), pairs in close proximity may be carrying diverse types of service using various modulation schemes, with considerable differences in signal levels (and receiver sensitivities) especially for pairs of considerably different lengths.
Statistical data has been developed that can be used to estimate crosstalk between services using different pairs of multi-pair telephone cables, for example in terms of BER (bit error rate) based on power spectral density (PSD, for example measured in milliwatts per Hertz expressed in decibels, or dBm/Hz) overlap between the services. However, this statistical data is of limited use in practice in the provision of a new service using equipment connected to a specific wire pair, in view of factors such as those discussed in the preceding paragraph.
It is therefore a significant concern of telephone companies that the signals and operation of existing systems may be adversely affected, especially as a result of NEXT, by the deployment of new equipment, particularly digital signal transmission equipment. This concern is increased in accordance with the extent to which such equipment is likely to be deployed, and hence particularly applies to equipment that may be used in very large numbers for remote access to computer networks. New equipment can be designed in a manner largely to avoid interference with other systems in accordance with the statistical data, but this imposes undesirable constraints on signal spectra and signal levels, limiting its usefulness in an unacceptable manner to accommodate a relatively small proportion of situations for which such constraints may be necessary.
An object of this invention is to provide a method that can permit new communications systems to be added to existing communications paths in a manner that is generally compatible with existing systems where these exist, and that can make optimum use of communications capacity.
This invention provides a method of determining a power spectral density (PSD) for supplying signals from a signal transmitter to a communications path, comprising the steps of: determining a PSD on the communications path, due to other communications, in the absence of signals supplied from the signal transmitter to the communications path; supplying signals from the signal transmitter to the communications path; and adjusting at least one parameter of the signals supplied from the signal transmitter to the communications path in dependence upon the determined PSD to reduce overlap between the PSD of the signals supplied from the signal transmitter to the communications path and the determined PSD.
Thus a new communications system, operating in accordance with this method, determines PSD on the communications path, primarily due to NEXT from other existing communications systems using adjacent communications paths, and then adjusts its own PSD to reduce, and desirably to minimize, overlap between the PSDs. On the basis that crosstalk between different communications paths is equal for opposite directions, the method consequently reduces, and desirably minimizes, interference from the new communications system with any existing communications systems that may be affected by the new system. Thus each new communications system that is deployed, for example in a public telephone network where the communications paths comprise twisted pair telephone lines, can be operated in a manner that is adaptively adjusted to minimize interference with existing systems in its own particular communications path environment. The adaptive adjustment can be performed only once on deployment of the new system, or much more desirably in an ongoing manner.
The step of determining a PSD on the communications path can comprise monitoring a PSD on the communications path, while signals are not supplied from the signal transmitter to the communications path, to produce the determined PSD. As an alternative to not supplying signals from the signal transmitter to the communications path during the monitoring, the PSD of signals supplied from the signal transmitter to the communications path could be subtracted from the monitored PSD representing the PSD on the communications path, due to other communications, in the absence of signals supplied from the signal transmitter to the communications path.
Thus the determined PSD can be constituted by the monitored PSD. Such a determination can be valid where the existing communications systems are symmetric systems, for which the PSD of signals having opposite directions of transmission can be substantially the same, but can be inaccurate for asymmetric systems for which the PSD of signals having opposite directions of transmission can be substantially different. For example, in an ADSL system the spectral utilization, and hence the PSDs, of signals in the two opposite directions of transmission are substantially different.
In view of this, preferably the step of determining a PSD on the communications path comprises the steps of: storing PSD templates for communications systems; monitoring a PSD on the communications path, due to other communications, in the absence of signals supplied from the signal transmitter to the communications path; comparing the monitored PSD on the communications path with the templates to identify a communications system corresponding to the monitored PSD; and producing the determined PSD in dependence upon the identified communications system. This enables the PSD of signals supplied from the signal transmitter to the communications path to be adjusted to reduce overlap with the PSD of signals of an existing system transmitted in the opposite direction of an adjacent communications path, this being appropriate because of the predominance of NEXT.
The at least one parameter that is adjusted to reduce PSD overlap can comprise the power (i.e. level), frequency band, and/or modulation scheme of signals supplied from the signal transmitter to the communications path. Desirably, all of these parameters are adjusted collectively to achieve minimal interference with existing communications systems consistent with optimal performance of the new communications system.